Topographic proxies from 2-D numerical analyses

We use 2D numerical analyses at ground motion stations from the NGA-West2 dataset to develop parameters to predict the effect of surface topography on response spectra. The simplistic numerical analyses use sinusoidal input motions, uniform soil profiles, elastic soil properties, and absorbing boundary conditions. We define several topographic parameters for stations using the natural logarithm of 2D amplifications in peak ground acceleration of a sinusoidal input motion in different orientations. The natural log of 2D amplifications when averaged over multiple orientations is found to have the most predictive power among the studied parameters. We also explore the relationship between the topographic parameters developed in this study, and the topographic parameters developed at the same sites in an earlier study (Rai et al. in Earthq Spectra, 2016b).     

Timing and genesis of base-metal mineralisation in black shales of the Upper Permian Ravnefjeld Formation,Wegener Halvø, East Greenland

Bituminous mud shales of the Upper Permian Ravnefjeld Formation (Zechstein 1 equivalent) are mineralised with zinc, lead and copper within a ca. 50 km² area on Wegener Halvø in central East Greenland. The occurrence of base-metal sulphides in shale nodules cemented prior to compaction indicates an early commencement of base-metal mineralisation. In other cases, post-compactional sulphide textures are observed. Homogeneous lead isotope signatures of galena and sphalerite from the shales (²⁰⁶Pb/²⁰⁴Pb: 18.440–18.466; ²⁰⁷Pb/²⁰⁴Pb: 16.554–16.586; ²⁰⁸Pb/²⁰⁴Pb: 38.240–38.326) suggest that all base metals were introduced during a single hydrothermal event. Therefore, post-compactional textures are believed to result from recrystallisation of early diagenetic sulphides during deep burial in the Upper Cretaceous to Tertiary. Lead isotope signatures of galena hosted in Upper Permian carbonate build-ups are relatively heterogeneous compared to those of the shale-hosted sulphides. The observed relations indicate a shared lead source for the two types of mineralisation, but different degrees of homogenisation during mineralisation. This suggests that lead was introduced to the carbonate rocks and black shales during two separate events. δ³⁴S of base-metal sulphides in the Ravnefjeld Formation lie between –12 and –4‰, whereas synsedimentary and early diagenetic pyrite in unmineralised shales in general have δ³⁴S between –47 and –16.5‰. Early diagenetic pyrite in the Wegener Halvø area in general has δ³⁴S 15 to 20‰ higher than the same pyrite morphotype in Triaselv in the western part of the basin. This relatively high δ³⁴S can be explained by extensive microbial sulphate reduction within persistent euxinic (super-anoxic) bottom waters under which supply of isotopically light seawater sulphate (and disproportionation of intermediate sulphur compounds) was restricted. The sulphur in the base-metal sulphides is believed to represent sulphide-dominated pore water, enriched in ³⁴S due to preferential removal of ³²S by sulphate-reducing bacteria and precipitation of diagenetic pyrite in the near-seafloor environment. We suggest that the sulphide-dominated pore water was trapped in the shale formation prior to introduction of base-metal-bearing fluids through fractures in the underlying carbonates, and that sulphide precipitation took place when the two fluids met. δ³⁴S values of carbonate-hosted base-metal sulphides fall within the same range as the shale-hosted ones. The relationship between barite and sulphides and evidence for pre-mineralisation entrapment of liquid hydrocarbons in the carbonates suggest that the sulphide in this case is derived by in-situ thermochemical sulphate reduction (TSR). Measured fractionation between sulphide and sulphate ranges from 18.5 to 24.4‰, suggesting temperatures of TSR around 70 to 100 °C. Vitrinite reflectance measurements in mineralised shale samples are all between 1.7 and 2.0%, except for samples taken close to a Tertiary dyke giving ca. 3.0%. Vitrinite reflectance data are comparable to previously published data from unmineralised shale samples in the area and could not be proven to correlate with the degree of mineralisation. This indicates that any early hydrothermal effect has been overprinted later, probably during deep burial in the Late Cretaceous to Early Tertiary as previously proposed.     

X-band low-grazing-angle ocean backscatter obtained during LOGAN1993

This paper reviews X-band ocean microwave backscatter data from the LOGAN (LOw Grazing ANgle) experiment conducted on the Chesapeake Light Tower by the Naval Air Warfare Center. The data were collected under varied wind, sea, and swell conditions that provide some new insights into low-grazing-angle backscatter phenomena. Transient backscatter peaks called “sea spikes” have long been associated with deep-water breaking waves; however, they have yet to be fully reconciled with backscatter and hydrodynamic theories. New analysis techniques have been applied to the LOGAN data that take advantage of the unique characteristics of sea spikes and their dynamics. High-resolution Doppler spectra are organized relative to the space-time centroids of the sea-spike clusters and conditionally averaged by RCS strength. The mean Doppler variation of the strongest sea spikes then map the breaking-wave structure just as Doppler histories measured at moderate grazing angles map the dynamics of the dominant linear surface-wave components. While breaking waves are manifest to some degree in backscatter data at all grazing angles, a non-Bragg-scatter mechanism accentuates the crest scattering at low grazing angles. The phenomena potentially can be exploited for remote ocean sensing and imaging     

On the unmodeled perturbations in the motion of uranus

In this paper we apply a numerical method to determine unmodeled perturbations in an attempt to explain the observed discrepancies in the motion of Uranus. We find that the estimated perturbation shows some significant periods that could be attributed to insufficient knowledge of the perturbations from some of the known planets. On the assumption that the gravitational attraction of an unknown planet is the origin of the deviations, the best planar solution of the inverse problem is a planet of 0.6 Earth masses, with true longitude of 133° (1990.5), semi major axis a = 44 AU and eccentricity e = 0.007.     

Organic carbon deliveries and their flow related dynamics in the Fitzroy estuary

The Fitzroy estuary (Queensland, Australia) receives large, but highly episodic, river flows from a catchment (144,000 km(2)) which has undergone major land clearing. Large quantities of suspended sediments, and particulate and dissolved organic carbon are delivered. At peak flows, delta(13)C (-21.7+/-0.8 per thousand) and C/N (14.8+/-1.3) of the suspended solids indicate that the particulate organic material entering the estuary is principally soil organic carbon. At the lower beginning flows the particulate organic matter comes from in-stream producers (delta(13)C=-26 per thousand). The DOC load is about 10 times the POC load. Using the inverse method, budgets for POC and DOC were constructed for high and low flows. Under high flows, only a small portion of the POC and DOC load is lost in the estuary. Under dry season (low flow) conditions the estuary is a sink for DOC, but remains a source of POC to the coastal waters.     

Formation of extremely F-rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin-granite magmas: a melt/fluid-inclusion study

Quartz crystals from topaz–zinnwaldite–albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200 m. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn–W mineralization. The combination of special experimental techniques with confocal laser Raman-microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H₂O, F, and B in re-homogenized MIs. The contents of H₂O and F were observed to increase from 3 to 30 and 1.9 to 6.4 wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H₂O, with values up to 55 wt% H₂O and an F concentration of approximately 3 wt%. Ongoing enrichment of volatiles H₂O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H₂O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid.

---

The development of volcanic hosted massive sulfide and barite–gold orebodies on Wetar Island, Indonesia

Wetar Island is composed of Neogene volcanic rocks and minor oceanic sediments and forms part of the Inner Banda Arc. The island preserves precious metal-rich volcanogenic massive sulfide and barite deposits, which produced approximately 17 metric tonnes of gold. The polymetallic massive sulfides are dominantly pyrite (locally arsenian), with minor chalcopyrite which are cut by late fractures infilled with covellite, chalcocite, tennantite–tetrahedrite, enargite, bornite and Fe-poor sphalerite. Barite orebodies are developed on the flanks and locally overly the massive sulfides. These orebodies comprise friable barite and minor sulfides, cemented by a series of complex arsenates, oxides, hydroxides and sulfate, with gold present as <10 m free grains. Linear and pipe-like structures comprising barite and iron-oxides beneath the barite deposits are interpreted as feeder structures to the barite mineralization. Hydrothermal alteration around the orebodies is zoned and dominated by illite–kaolinite–smectite assemblages; however, local alunite and pyrophyllite are indicative of late acidic, oxidizing hydrothermal fluids proximal to mineralization. Altered footwall volcanic rocks give an illite K–Ar age of 4.7±0.16 Ma and a ⁴⁰Ar/³⁹Ar age of 4.93±0.21 Ma. Fluid inclusion data suggest that hydrothermal fluid temperatures were around 250–270°C, showed no evidence of boiling, with a mean salinity of 3.2 wt% equivalent NaCl. The ³⁴S composition of sulfides ranges between +3.3 and +11.7 and suggests a significant contribution of sulfur from the underlying volcanic edifice. The ³⁴S barite data vary between +22.4 and +31.0, close to Miocene seawater sulfate. Whole rock ⁸⁷Sr/⁸⁶Sr analyses of unaltered volcanic rocks (0.70748–0.71106) reflect contributions from subducted continental material in their source region. The ⁸⁷Sr/⁸⁶Sr barite data (0.7076–0.7088) indicate a dominant Miocene seawater component to the hydrothermal system. The mineral deposits formed on the flanks of a volcanic edifice at depths of ~2 km. Spectacular sulfide mounds showing talus textures are localized onto faults, which provided the main pathways for high-temperature hydrothermal fluids and the development of associated stockworks. The orebodies were covered and preserved by post-mineralization chert, gypsum, Globigerina-bearing limestone, lahars, subaqueous debris flows and pyroclastics rocks.     

TBT contamination identified in Antarctic marine sediments

We report for the first time butyltin contamination of near-shore sediments at six sites in the Ross Sea, Antarctica. A very high concentration of 2290 microg Sn kg(-1) sediment was recorded in one sample. The most likely source is abrasion of antifouling paint from the hulls of ice-breakers, but this pattern of contamination is also possible following ship groundings. Antifoulant biocides, such as TBT, have not been considered or detected in Antarctica previously and represent a new challenge to environmental managers and custodians.